Add :json type to auto_discovery_link_tag

This allows auto_discovery_link_tag to support the JSON Feed standard.
See https://jsonfeed.org/version/1 for more information.

Fix `warning: assigned but unused variable - timestamp`

Add documentation to accessors in AbstractController::Base

Suppress `warning: assigned but unused variable - stdout`

This retains the existing behavior of
ActiveSupport::Cache.expand_cache_key (as used by etaging) where the
cache key includes the version.

see: https://bugs.ruby-lang.org/issues/6696

* Fix indentation.
* Add backticks.

[ci skip] explain async queue and rake tasks

Fixed Frozen string error in actionpack

Remove unneeded `association.respond_to?(:reset_scope)`

CI against JRuby 9.1.9.0

http://jruby.org/2017/05/16/jruby-9-1-9-0

Since 86390c3b all associations have `reset_scope` so `respond_to?` is
unneeded.

Set non 0 value to logger not to be affected by Ruby versions

Address LogSubscriberTest failures to support Rails 2.5.0-dev

Addresses #29021

The object inside the scope is of the class that define the association
not the associated class.

[ci skip]

Added fixed links to code for Rails Guides ActiveSupport

Address #29021

Since Ruby 2.5.0-dev does not return decimal value when it is 0.
This change has been made at Ruby 2.5.0-dev between `(2017-05-05 trunk 58572)`
and `(2017-05-07 trunk 58586)`, likely Revision 58586.

This fix has been tested with these Ruby versions:

* ruby 2.5.0dev (2017-05-15 trunk 58733) [x86_64-linux]
* ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux]
* ruby 2.3.4p301 (2017-03-30 revision 58214) [x86_64-linux]
* ruby 2.2.7p470 (2017-03-28 revision 58194) [x86_64-linux]

[Yasuo Honda & Ryuta Kamizono]

Fix `TestInvalidUrls` with rack 2.0.3

Respect 'SchemaDumper.ignore_tables' in databases structure dump

* Use NOT IN in SQL query
* Quote table names propertly
* Use array form of command invocation

When using `sql` as the schema format, or even just doing `rake
db:structure:dump`, it would be good to respect the list of ignored
tables that has been configured.

Currently, raise `BadRequest` if params encoding is invalid.
https://github.com/rails/rails/blob/5-1-stable/actionpack/lib/action_dispatch/http/parameters.rb#L64..L74
https://github.com/rails/rails/blob/5-1-stable/actionpack/lib/action_dispatch/request/utils.rb#L26..L39

However, env values are ensure encoded in ASCII 8 BIT at rack 2.0.3.
https://github.com/rack/rack/commit/68db9aa99e3e2775a58621f658b2a7a0f67db459

Therefore, even if specify an invalid urls, it will not cause an error.